The present invention generally relates to semiconductor devices and more particularly to a high-speed compound semiconductor device having an LDD (lightly doped drain) structure in combination with a low-resistance electrode provided on a gate electrode.
A compound semiconductor device having a gate electrode of a refractory metal has an advantageous feature of high operational speed and is used extensively in compound semiconductor integrated circuits for use in ultra-high frequency applications such as mobile telephony. A representative example of the mobile telephony includes portable telephones. Further, such a compound semiconductor device is advantageous in that the diffusion region can be formed relatively easily by a self-alignment process. Further, the feature of using a refractory metal for the gate electrode is advantageous in conducting a thermal diffusion process for forming a diffusion region.
Meanwhile, with the development of telecommunication technology, the analog devices and digital devices used in such ultra-high frequency applications are required to have the feature of higher operational speed and lower power consumption.
The desired increase of the operational speed is most conveniently achieved by reducing the gate length of the compound semiconductor device. On the other hand, such a decrease of gate length invites the problem of increase in the gate resistance as a result of the decrease of the cross-sectional area of the gate electrode associated with the device miniaturization. While it is generally practiced in the art to use a refractory metal silicide such as WSi for the material of the gate electrode in view of high-quality Schottky contact formed between the refractory metal silicide and a compound semiconductor substrate and further in view of the refractory nature of the refractory metal silicide, it is nevertheless desired to reduce the resistivity of the refractory metal silicide gate electrode further. It should be noted that WSi has a relatively large resistivity of about 2.times.10.sup.-4 .OMEGA..cm. A similar situation holds also in other silicides of refractory elements. Further, in such a high-speed semiconductor device having a reduced gate length, it is desired to employ the so-called LDD structure for suppressing the problem of short channel effect.
In order to reduce the gate resistance in such high-speed compound semiconductor devices, there is a proposal to construct the gate electrode in two layers, the first layer being a Schottky electrode layer and the other layer being a low-resistance layer. In one example, it is proposed to form the low-resistance layer on a WSi Schottky electrode layer after conducting a thermal annealing process such as a thermal diffusion process. Alternatively, there is a proposal to deposit a WSi Schottky electrode and a low-resistance layer consecutively to form a substantially single electrode having a multiple layer structure.
In the former approach, it is possible to use a low-melting metal such as Au for the low-resistance layer. On the other hand, this former approach has a drawback in that the number of fabrication steps of the semiconductor device is increased substantially as a result of the addition of the process of forming the low-resistance layer separately from the process of forming the Schottky electrode.
In the latter approach, on the other hand, it is possible to form the multiple-layer gate structure by a single patterning process and the fabrication of the semiconductor device is conducted similarly to the case of using a single-layer gate electrode. On the other hand, the latter approach has a drawback in that there tends to occur a thermal stress in the gate electrode as a result of the difference in the thermal expansion coefficients between the Schottky electrode and the low-resistance layer, while such a thermal stress induces a deterioration in the FET performance of the device. Further, the thermal stress tends to cause an exfoliation of the low-resistance electrode layer from the Schottky electrode layer.
In order to overcome the foregoing problems of the latter approach, there is a further proposal, as disclosed in the Japanese Laid-Open Patent Publication 2-234442, to interpose a stress-relaxation layer between the low-resistance electrode layer and the Schottky electrode layer.
FIGS. 1A-1D show the fabrication process of a MESFET according to the disclosure of the foregoing Japanese Laid-Open Patent Publication 2-234442.
Referring to FIG. 1A, a GaAs layer 2 of the n-type is formed on a semi-insulating GaAs substrate 1 by an MOVPE process or an MBE process, and a Schottky electrode layer 3 of WSi and a stress-relaxation layer 4 of LaB.sub.6 are deposited consecutively on the GaAs layer 2. Further, a low-resistance layer 5 of W is deposited on the stress-relaxation layer 4.
Next, in the step of FIG. 1B, the low-resistance layer 5 of W is patterned to form a W pattern designated hereinafter by the reference 5, on the stress-relaxation layer 4, and an ion implantation process of Si.sup.+ is conducted, in the step of FIG. 1C, into the GaAs substrate 1 through the LaB.sub.6 stress-relaxation layer 4 and further through the WSi Schottky electrode layer 3, while using the W pattern as a mask. The Si ions thus introduced are activated by a thermal diffusion process, wherein the thermal diffusion process is conducted in the state that the WSi layer 3 and the LaB.sub.6 layer 4 cover the substrate 1. As a result of the thermal diffusion process, diffusion regions 6 of the n-type are formed in the substrate 1.
In the step of FIG. 1D, the stress-relaxation layer 4 of LaB.sub.6 is patterned by an ion milling process using Ar, and the WSi Schottky electrode layer 3 is patterned by a dry etching process while using the LaB.sub.6 stress-relaxation layer 4 and the W low-resistance layer 5 thereon as a mask. Further, ohmic electrodes 7 are formed in ohmic contact with the diffusion regions 6 at both lateral sides of the gate structure that includes the stacking of the WSi Schottky electrode layer 3, the LaB.sub.6 stress-relaxation layer 4 and the W low-resistance layer 5.
In the conventional process of FIGS. 1A-1D, on the other hand, there arises a problem in that the formation of the LDD structure in the substrate 1 is difficult.
In the process of FIGS. 1A-1D, it should be noted that an n.sup.+ -type region is formed in the p-type substrate 1 in correspondence to the source and drain regions when forming an n-channel FET, while it should be noted that such a structure, characterized by a sharp change of impurity concentration level between the p-type region of the substrate 1 and the diffusion region 6, tends to suffer from the problem of hot carrier formation during the FET operation as a result of the large electric field induced in the vicinity of the drain edge. When this problem occurs, the operational performance of the FET is deteriorated inevitably.
In order to overcome the foregoing problems, it is practiced to use an LDD structure in the FETs having such a short gate length, wherein an LDD structure includes an n.sup.- -type region having a reduced impurity concentration level between the p-channel region of the substrate 1 and the n.sup.+ -type diffusion region 6 for relaxing the concentration of the electric field. Such an LDD structure is essential in such a high-speed FET having a short gate length for suppressing the short channel effect.
In the foregoing conventional process, on the other hand, the impurity ions are injected into the substrate 1 through the WSi Schottky electrode layer 3 and the LaB.sub.6 stress-relaxation layer 4. In such a process, it should be noted that the process window for an appropriate acceleration energy of ion implantation is extremely small, and a minute deviation in the film thickness of the layers 3 and 4 induces a substantial deviation in the thickness of the junction formed in the substrate 1. This means that the foregoing conventional process is not suitable for forming a shallow n.sup.- -type diffusion region used in the LDD structure.
In order to form a shallow diffusion region, it is necessary to reduce the thickness of the layer 3 or 4 as much as possible, while there arises a problem that the desired high-quality Schottky electrode is not obtained when the thickness of the WSi layer 3 is reduced. When the thickness of the LaB.sub.6 stress-relaxation layer 4 is reduced, on the other hand, the effect of the stress-relaxation becomes poor and inefficient.
In the conventional fabrication process, it should also be noted that the gate width of the gate structure including the W layer 5 becomes small when the gate length is reduced to submicron or quarter submicron lengths. When this occurs, the cross-sectional area of the gate structure becomes also small and there occurs an increase of the gate resistance even in such a case in which the gate structure includes the W low-resistance layer 5. In other words, the effect of the W low-resistance layer 5 for reducing the gate resistance is diminished.
In order to overcome this problem, it is practiced to form the gate structure to have a T-shaped form in which the low-resistance layer 5 forms an overhang structure extending laterally on the Schottky electrode layer 3.
When this T-shaped gate structure is to be formed in the conventional process of FIGS. 1A-1D, it is necessary to conduct a lateral or side etching of the WSi layer 3 selectively with respect to the rest of the layers 4 and 5. Because of the insufficient selectively of etching rate between the WSi layer 3 and the W layer 5, however, it is difficult to form the desired T-shaped gate structure according to the foregoing conventional process.
Further, even when the T-shaped gate structure is formed successfully by the lateral etching process of the WSi layer 3, the gate length of such a T-shaped gate structure tends to vary variously, while such a change in the gate length induces a variation of the threshold voltage of the FET. As the control of the gate length by the lateral etching is difficult, the control of the threshold voltage is also difficult.
In addition, the foregoing conventional fabrication process has a further drawback, when mass-producing FETs, in that the inspection of the gate length of the fabricated device is difficult. It should be noted that there is no way to inspect the gate length during the fabrication process of the device and the confirmation of the gate length of the actual device is only possible by a microscopic observation of the cross-section conducted for a completed semiconductor device.
It is generally known that a Schottky junction tends to show a poor performance when the impurity concentration level in the semiconductor layer is high. Because of this, it is practiced in conventional MESFETs to improve the forward-bias performance of the Schottky junction and to improve the reverse withstand voltage thereof by providing a separation between the gate electrode and the n.sup.- -type region forming an LDD structure together with the n.sup.+ -type region. However, the foregoing conventional process, which relies on the injection of impurity ions via the LaB.sub.6 stress-relaxation layer 4 and further through the WSi Schottky electrode layer 3, is not suitable for exact control of the lateral spreading of the injected impurity ions, and it has been difficult to secure the necessary performance for the Schottky electrode.